1. Field of the Invention
The present invention relates to a scale and an optical encoder.
2. Description of Related Art
An optical encoder including a scale and a detection head that moves along the scale has been used, for example, for measurement of a moving distance in a manufacturing device. Japanese Patent No. 4274751 discloses an optical encoder including a scale in which displacement detection patterns (main-signal scale gratings) and reference position detection patterns (reference-signal scale gratings) are arranged in parallel in a direction perpendicular to a length measurement axis.
As shown in FIG. 12, an optical encoder 7 disclosed in Japanese Patent No. 4274751 includes a scale 70 and a detection head 80 that relatively moves along the scale 70. FIG. 13 is a plane view of the scale 70.
As shown in FIG. 13, in the scale 70, displacement detection patterns 71 and a reference position detection pattern 72 are arranged in parallel in a direction (Y-axis direction) perpendicular to a length measurement axis (X-axis direction). In the displacement detection patterns 71, transmitting parts 73 and non-transmitting parts 74 are alternately arranged along the length measurement direction (X-axis direction) of the scale 70.
As shown in FIG. 12, the detection head 80 includes a light source 31, light source gratings 81 arranged between the scale 70 and the light source 31, and a light receiving part 82 arranged in a position opposed to the light source grating 81 with the scale 70 interposed therebetween. In the light receiving part 82, receiver gratings 83 and a reference-signal receiver element 84 are arranged in parallel to each other.
The light source 31 emits light to the light source gratings 81, and the light that has passed through the light source gratings 81 is made incident on the scale 70. The light that has been made incident on the scale 70 is diffracted by the displacement detection patterns 71 and the reference position detection pattern 72, a main-signal interference fringe is generated by the displacement detection patterns 71, and a reference-signal interference fringe is generated by the reference position detection pattern 72. The main-signal interference fringe is detected by the receiver gratings 83 and the reference-signal interference fringe is detected by the reference-signal receiver element 84.
The optical encoder 7 disclosed in Japanese Patent No. 4274751 reads an increase and a decrease in the luminance of the main-signal interference fringes when the detection head 80 moves along the scale 70 to measure the number of main-signal interference fringes by which the detection head 80 has moved from a measurement start position. Further, the reference-signal receiver element 84 detects the reference-signal interference fringe to set the position at which the reference-signal interference fringe has been detected as a reference position for measuring a moving distance. In this way, it is possible to measure the current position of the detection head 80 based on the reference position and the moving distance from the measurement start position.